Transmitter



July 2, 1940. J, H WASSELL 2,205,.fi82

TRANSMITTER Filed Aug. 27, 1957 INPU T Mam/arm mmM/rrm INVENTOR HAROL D J. H. WASRSELL BY wm ATTORNEY v Patented July 2, 1940 PATENT OFFCE TRANSMITTER Harold Joseph "Hughes Wasscll, Chelmsford,

. England; assignor to Radio Corporation of America, a corporation of Delaware Application August 27, 1937, Serial No. 161,271

In Great Britain October 21, 1936 j j a Claims. (01. 250- 17 I This invention relates to radio and other modulatedcarrier wave transmitters, and more particularly to carrier wave transmitters of the socalled suppressed carrier type by'which expression as employed in this specification is meant a transmitter wherein the carrier is automatically switched off inthe absence of modulating signals.

In the usual known transmitters of the suppressed carrier type a direct current voltage is de rived by rectifying speech or other modulating signals and this voltageis utilized either directly or after amplification for the purpose of putting the carrier on and 01f. In order to prevent the carrier being put oif during inter-syllabic pauses it is usual to provide a'time delaycircuit to cause the speech-derived direct current voltage to ceased. The time delay circuit usually employed for this purpose provides for a slow decay of the direct current voltage-generallyin logarithmic fashion-and the circuit as a whole is so arranged that the carrier is not entirely off until the direct current voltage has reached a predetermined low value. A disadvantage of a transmitter as above described is that the carrier becomes of very low value (as compared to normal carrier level) just before it is entirely off and during these periods small alternating potentials-for exampleQpOtentialsarising by reason of the use of jthermiom'c tubes with filaments energized'byraw alternating currentmay give rise to undesired and relatively deep modulation of the reduced carrier with the result that objectionable noises are set up at the ends of speech periods.

The object of the present invention is to provide improved transmitters of the kind referred to wherein the above disadvantage is greatly reduced or avoided without at the same time giving rise to serious clipping of speech.

According to this invention a direct current potential derived by rectifying speech or other modulating signals is employed to control a relay which in one position completes a circuit for connecting an additional source of potential in such manner as to maintain the transmitter in the carrier condition the arrangement being such that this source of potential is left in circuit in this manner until the controlling potential has become reduced to a predetermined low value whereupon the relay changes the circuit connections so as effectively to switch out the said source and thus suddenly cut oif the carrier.

The invention is illustrated in the accompanying drawing which shows diagrammatically two embodiments.

Referring to Figure 1, speech or other modulating potentials derived from any suitable point in the transmitter I0 are fed to the primary winding of a transformer T whose secondary winding is shunted by a circuit consisting of a rectifier Re in 5 series (in the order stated) ;with a resistanceRi and with the operating winding X of a sensitive electro-magnetic relay. A condenser C1 is shunted between the junction point of the rectifier Re with the resistance R1 and the junction point of the operating winding X with the secondary winding of the transformer T, the latter junction point being grounded. The former junction point is also connected to the rest contact a of the electron-magnetic relay (1. e. that 15 contact on which the relay armature Ra rests hang on for ashort period after speechhas when the windingX is not excited) and the other or energized contact b of the electro-magnetic relay is connected to ground through a source of potential E1 the positiveterminal of which is grounded. The transmitter I0 proper is keyed on and ofi as regards carrier by meansof a thermionic tube V1 connected in the transmitter proper in any maner well known per se and serving to effect control'in dependence upon its grid potential. The cathode of the tube V1 is grounded and the control grid thereof is connected to the said cathode through a suitable grid resistance R2 and is also connected to the armature Ra of the relay.

The operation of the above arrangement is as follows:

In the absence of speech input the relay is not energized and its armature remains upon the rest contact a. When speech commmences a 35 direct current potential is developed across the condenser C1 and by reason of the position of the relay armature Ra direct current potential is also set up across the grid resistance R2. The carrier is at once switched on and after a very small fraction of time the condenser is changed and the potential across the condenser rises sufficiently for the relay, which is sensitive enough to operate at a small fraction of the current which flows through its winding X during speech periods, to 45 move its armature over to the energized contact I), thus switching in the source of potential E1 and maintaining the transmitter ID in the radiating condition. When speech input ceases the direct current voltage across the condenser C1 50 decays relatively slowly until it reaches a predetermined low value at which the relay armature Ra returns to the rest contact whereupon the negative potential from the source of potential E1 is suddenly removed from the grid of the tube and the carrier is accordingly suddenly keyed off. In this way gradual decay of the carrier through values at which it is likely to be subjccted to relatively deep modulation by hum and other interfering potentials is avoided.

In the modified circuit shown in Figure :2 the resistance R1 in series with the rectifier Re is directly shunted by the condenser C1 and a preferably adjustable tapping point P upon this resistance is connected to the control grid of an interposed additional tube V2 whose cathode is grounded and connected to the junction point of the transformer secondary, with the resistance R1. The junction point of the rectifier Re with the resistance R1 is connected to the energized contact I) of the relay and the rest contact a is grounded through a source of potential E1 whose positive terminal is again on the ground side. The grid of the tube V1 by which keying of the carrier is efiected is connected to the armature Ra of the relay and is also connected through a grid resistance R2 to the grounded cathode of the said tube. The relay winding X is included in series with a source E2 of suitable anode potential between the anode and cathode of the interposed additional tube and a by-pass condenser K is provided between that end of the relay winding remote from the anode and the cathode.

The purpose of the additional interposed tube in the modified construction above described- (this is the preferred construction)is to render the power required from the time delay circuit consisting of the resistance and condenser in the rectifier circuit less than is the case with the first described embodiment.

- I'claimz -1. In a signalling system, a transmitter from which a carrier wave modulated by signal potentials is to be transmitted, and means to provide a carrier or suppress the same in the presence and absence of signal potentials respectively comprising an electron discharge tube having electrodes including a control grid and a cathode with a pair of its electrodes connected to said transmitter to cause it to provide said carrier or suppress the same, a first contact, a source of potential having a positive terminal connected to the cathode of said tube and a negative terminal connected with said first contact, a second contact, a switch connected with the grid of said tube and cooperating with said first contact to supply to said grid a potential from said source such that said tube causes said transmitter to provide a carrier, said switch also cooperating with said second contact to supply to said grid a potential which causes said transmitter to suppress said carrier, means connected to said second contact to supply to said control grid by way of said switch a potential which causes said transmitter to suppress said carrier in the absence of signal potentials, and a second means associated with said switch to move the same in conductive relation with one of said contacts in the presence of signal potentials and with the other contact in the absence of signal potentials.

2. In a signalling system, a transmitter from which a carrier wave modulated by signal potentials is to be transmitted, and means to provide a carrier or suppress the same in the presence and absence of'signal potentials respectively comprising an electron discharge tube having electrodes.

including, a control grid and a cathode with its electrodes connected to said transmitter to cause said transmitter to provide the carrier or sup- 1 press the same, a first contact, a source of potential having a positive terminal connected to the cathode of said tube and a negative terminal connected with said first contact, a second contact, a relay armature connected with the grid of said tube and cooperating with said first contact to supply to said grida potential such that said tube causes said transmitter to provide a carrier, said armature also cooperating with said second contact to supply to said grid a potential which causes said transmitter to suppress said carrier, a rectifier having an input energized by said signal potentials, said rectifier having an output, and a relay winding connected to the output of said rectifier and associated with said armature to move the same in contact with one of said contacts in the presence of signal potentials and in contact with the other contact in the absence of signal potentials.

3. A system as recited in claim. 2 wherein an electron discharge tube amplifier is coupled at its input to said rectifier and at its output to said relay winding to amplifythe currents therein which control the position of said armature.

HAROLD JOSEPH HUGHES WASSELL. 

